Hello,
The size guide is a custom-type post that is why it is shown in the menu in the same way as HTML block, however, they are not for the menu. We do not remove these items as it requires accessive code what we try avoid.
You can find and add size gied element in the builder chart. You can try to add to HTML block and then show it as a mega menu item: here are the instruction for:
Size guide: https://xtemos.com/docs/woodmart/faq-guides/create-size-guide-table/
HTML block: https://xtemos.com/docs/woodmart/html-blocks-2/
Megamenu: https://xtemos.com/docs/woodmart/create-menu-2/
If you have any questions please feel free to contact us.
Best Regards